In the implementation I provided, you can see how it follows the structure outlined in the RFC, though in a simplified manner:

Setup

The RFC describes server setup with server_private_key, server_public_key, and an oprf_seed. In my code:

def __init__(self):
    # In a real implementation, this would be securely stored
    self.server_private_key = os.urandom(32)

Then for each user:

# 1. Server generates a random OPRF seed for this user
oprf_seed = os.urandom(OPRF_SEED_LEN)

Registration

The RFC describes registration needing server authentication, exchange of messages, and resulting in:

1. Client receiving an export_key
2. Server storing a record

In my implementation:

def register_user(self, username, password):
    """Register a new user with the OPAQUE protocol"""
    # 1. Server generates a random OPRF seed for this user
    oprf_seed = os.urandom(OPRF_SEED_LEN)
    
    # 2. Client and server engage in OPRF protocol
    # (In reality, this would be an interactive protocol)
    blind, blinded_element = self._client_blind(password)
    evaluated_element = self._server_evaluate(oprf_seed, blinded_element)
    oprf_output = self._client_finalize(password, blind, evaluated_element)
    
    # 3. Client derives envelope encryption key
    envelope_key = self._derive_key(oprf_output, b"EnvelopeKey")
    
    # 4. Client creates an "envelope" that protects their authentication data
    # In a real implementation, this would include client credentials
    auth_data = os.urandom(32)  # Simulated auth data
    envelope = self._create_envelope(envelope_key, auth_data)
    
    # 5. Server stores user record
    user_record = {
        "username": username,
        "oprf_seed": oprf_seed,
        "envelope": envelope,
        # No password-equivalent material is stored
    }
    
    return user_record

The major simplification in my code is that the registration protocol exchange (RegistrationRequest, RegistrationResponse, RegistrationRecord) is collapsed into a single function call rather than showing the actual message passing between client and server.

In a full implementation, you would see:

1. The client sending a blinded password value (RegistrationRequest)
2. The server responding with its evaluation (RegistrationResponse)
3. The client creating and sending back an envelope (RegistrationRecord)

My implementation also doesn't explicitly return an export_key to the client, which is mentioned in the RFC as a potential output for encryption of additional data.

For a more directly RFC-compliant implementation, you would need to:

1. Implement the actual request/response protocol
2. Include the AKE key exchange functionality
3. Return the export_key
4. Follow the exact message formats specified in Section 5.1

Here are key elements from the RFC that are missing or simplified in my code:

1. Online Authenticated Key Exchange (3.3): The full OPAQUE protocol includes a complete authenticated key exchange phase, which is only partially implemented in my code (it creates a session key but doesn't include the full AKE protocol).

2. Envelope Structure (4.1.1): The RFC specifies a particular structure for the envelope, whereas my code uses a simplified version.

3. 3DH Protocol (6.4): The code doesn't implement the three Diffie-Hellman exchanges that are part of the full OPAQUE specification.

4. Message Formats (5.1, 6.1): The proper wire formats for messages aren't implemented.

5. Key Schedule Functions (6.4.2): The specific key derivation schedule is simplified.

The implementation I provided is more of a conceptual sketch showing the core OPAQUE principles (OPRF + envelope encryption) rather than a complete implementation of the RFC. It demonstrates the password-hardening and zero-knowledge aspects but doesn't include the full authenticated key exchange.